Evolution and Ecology Lab (BIOL 3113)

Spring 2003


Artificial Selection and Evolution: Setup

Click here to access the pre-lab questions for this lab.


Introduction

Over hundreds or even thousands of years, humans have altered various species of plants and animals for our own use by selecting individuals for breeding that possessed certain desirable characteristics, and continuing this selective breeding process generation after generation. Thus, all domestic dog varieties, from chihuahuas to great danes, trace their separate lineages to a common wild ancestor, the wolf (Canis lupus). Similarly, a wide variety of familiar and highly nutritious vegetables originate from just a few species of wild mustards, in particular Brassica rapa, B. oleracea, and B. juncea.

That plant and animal breeders have been able to dramatically change the appearance of various lineages of organisms in a relatively short period of time is an obvious yet profound fact. Charles Darwin used many examples of selection by humans to help support the case for his proposed mechanism resulting in evolution of natural populations; i.e., natural selection.

Many studies on many kinds of organisms have shown that most variable traits respond to artificial selection (i.e., it is usually possible, even easy, to increase or decrease the frequency or average value of a trait in a lineage through careful selective breeding). Starting today in a lab exercise, you will attempt to accomplish the same thing.

Artificial selection vs. natural selection

Natural selection is a deceptively simple concept, relatively easy to understand at a basic level, but with profound implications that are intellectually challenging. The following exercise in artificial selection will serve as an introduction to natural selection, and we hope will help you to better understand this important concept. Fundamentally, artificial selection and natural selection are quite similar, but there are a few important differences.

The definition of natural selection is simply differential survival and reproductive success. In other words, not all individuals in a population leave the same number of offspring. This arises because individuals vary in many traits that affect their ability to survive and reproduce in a given environment. When this variability has a genetic basis (i.e., is inheritable-passed on from parents to offspring), natural selection can lead to evolutionary change in the expression of the trait in the population.

Artificial selection is essentially this same process, except that favored traits are those that for one reason or another are preferred by humans, rather than those that enhance the organism's fit to its environment. Artificial selection generally is much faster than natural selection, because the next generation can be absolutely restricted to offspring of parents that meet the desired criteria (rarely is natural selection such an all-or-none phenomenon).

An Artificial Selection Experiment Using Wisconsin Fast Plants

Normally, natural selection, and even artificial selection, occurs much too slowly to directly observe in a classroom setting. However, Wisconsin Fast Plants, a variety of Brassica rapa (the same species as turnip, bok choi and Chinese cabbage) has a remarkably short life cycle (6-7 weeks from seed to seed under lab conditions), which should let us study one complete generation this semester. Fast Plants are a product of 20 years of intense artificial selection by researchers at the University of Wisconsin for the following traits: rapid flowering and maturation; high seed production; short stature; and the ability to thrive under artificial light. The result of these efforts has been a valuable research and educational tool.

The lab you set up today will run much of the length of the semester. The basic goal of this lab is to see whether intense artificial selection on a single trait in a parental generation will cause that trait to evolve in the offspring generation. Today you will plant the seeds of the parental generation of Fast Plants, all from the same basic stock of "wild type" Fast Plants obtained from a commercial source.

  • From the front desk, each bench group should collect: 20 plastic film canisters (which serve as planters for individual seeds); 20 blue diamond wicks; a beaker full of soil; a vial of 20 seeds; lab tape.

  • Insert a wick into the hole in the bottom of each canister

  • Fill the canister loosely with potting soil level with the top.

  • Spray the canisters thoroughly with water.

  • Plant one seed in the middle of each canister. Be sure to plant the seed just below the surface of the soil. Do not stick it deep into the soil, otherwise the seed will not germinate.

  • Write your lab time, bench number, and plant ID number (1-20) on label tape with a Sharpie, and put one on each canister

After planting, your canisters will be placed under the continuous light bank in the lab room, and maintained by the instructors in an automatic watering system. By the end of this week, most of the seeds will have germinated. In two weeks they will be ready for the next phase of the experiment.

Acknowledgements: This lab was derived from one developed by Dr. Bruce A. Fall of the University of Minnesota.


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